1. Field of the Invention
This invention relates to quality control testing of electronic circuitry and, more particularly, to equipment and methods for the non-destructive testing of integrated circuits, using a focused electron beam in making current/voltage measurements on photodiode arrays.
2. Description of Related Art
The development of photodiode arrays comprising large numbers of individual elements is becoming increasingly important in certain specific applications. For quality assurance, it is virtually essential that screening of a fabricated photodiode array on an individual element basis be performed prior to incorporation of the array in utilization equipment. Heretofore, such screening has been performed by mechanical probing of the elements, usually on a single element basis. The results of such a procedure are less than satisfactory. Delicate indium bumps which are provided for circuit test contacts are easily damaged by a contacting probe. The procedure is a very time-consuming operation. A further disadvantage is that the results of the procedure are extremely operator dependent.
Given the nature of the problem--small detector size (approximating 1.5.times.1.5 mils) of typical multi-element hybrid focal plane arrays and the delicate character of the indium interconnects--some type of non-contacting method of diode probing is desired, preferably one which can be automated or at least performed in a way which provides results independent of operator parameters.
A system has been developed which uses an electron beam integrated circuit tester for testing the internal nodes of a complex integrated circuit. This system incorporates electron beam apparatus which focuses and directs the electron beam to selected internal nodes and detects secondary electron emission therefrom with associated computerized control circuitry. As thus controlled, the electron beam apparatus provides high speed access and testing of the integrated circuit nodes.
In one version of the system, the probe intelligently chooses a limited number of the internal nodes which are considered most likely to indicate circuit failure so as to minimize the number of nodes tested while maximizing reliability of the results. The probe comprises an artificial intelligence which understands the design and operation of the integrated circuit under test, the intelligence being embodied in a programmed computer associated with the probe. The system further includes circuitry for accessing the peripheral pads of the integrated circuit under test and for applying the proper circuit biases, clock signals and test signals under control of the computer. In such a system, the movement of the electron beam between selected internal nodes of the circuit and the operation of the associated computer to select those nodes are decisions which may be made in a matter of microseconds or milliseconds.
The electron beam of the test apparatus, when focused on a single selected node within the circuit, creates secondary electron emission having an energy distribution which is affected by the properties of the node on which the electrons of the primary beam impinge. A suitable sensor responds to the secondary electron energy distribution, permitting the computer to sense and store the voltage of the node under test.
A particular advantage of such a system is that the probe does not capacitively load any of the internal nodes of the integrated circuit under test, nor does it damage them. At the same time, the electron beam of the probe has a submicrometer diameter and is fast and easy to position with great precision. Because of these advantages, use of this system as an off-line design verification and failure analysis tool for integrated circuit development is one practical application. For photodiode probing, the high speed with which the nodes may be selected and tested is appropriate for the use of the system on a production line having high product through-put. This method is cost effective and is inherently more reliable than the conventional testing methods outlined hereinabove.